Power transfer systems of the type used in motor vehicles including, but not limited to, four-wheel drive transfer cases, all-wheel drive power take-off units (PTU), limited slip drive axles and torque vectoring drive modules are commonly equipped with a torque transfer mechanism. In general, the torque transfer mechanism functions to regulate the transfer of drive torque between a rotary input component and a rotary output component. More specifically, a multi-plate friction clutch is typically disposed between the rotary input and output components and its engagement is varied to regulate the amount of drive torque transferred from the input component to the output component.
Engagement of the friction clutch is varied by adaptively controlling the magnitude of a clutch engagement force that is applied to the multi-plate friction clutch via a clutch actuator system. Traditional clutch actuator systems include a power-operated drive mechanism and an operator mechanism. The operator mechanism typically converts the force or torque generated by the power-operated drive mechanism into the engagement force which, in turn, can be further amplified prior to being applied to the friction clutch. Actuation of the power-operated drive mechanism is controlled based on control signals generated by a control system.
The quality and accuracy of the drive torque transferred across the friction clutch is largely based on the frictional interface between the interleaved clutch plates of the clutch pack. When partially engaged, the clutch plates slip relative to one another, thereby generating heat. As such, lubricating fluid must be directed through and around the clutch pack to cool the clutch plates. Excessive heat generation, however, can degrade the lubricating fluid and damage the friction clutch components. Additionally, electronic traction control systems require the clutch control system to respond to torque commands in a quick and accurate manner. The accuracy required to such a torque request is largely dependent on the coefficient of friction of the clutch pack. It has been demonstrated that the coefficient can change quite rapidly under various loading and/or slip conditions. The coefficient tends to fade due to significant temperature rise in the clutch pack, resulting from insufficient heat removal. The heat removal rate is dependent upon lubricating fluid flow rate and condition of the lubricating fluid.